Process for restoring or maintaining the activity of heterogeneous catalysts for reactions at normal and low pressures

ABSTRACT

By the process of the present invention the activity of heterogeneous catalysts for reactions at normal and low pressures is restored or maintained. These catalysts can lose their activity as a result of the deposition, physisorption or chemisorption of organic or inorganic deactivating substances which are carried over into the reaction system or are formed as the result of side reactions in a chemical synthesis carried out with the aid of the catalyst. The activity is restored or maintained by using a pressure greater than the critical pressure of the fluid phase and a temperature higher than or equal to the critical temperature of the fluid phase. In this process, the pressure and temperature are applied for a period such that the deactivating substances are either removed from the catalyst or are not initially deposited thereon or absorbed or formed.

This application is a division of application Ser. No. 731,348, filedMay 6, 1985, now U.S. Pat. No. 4,605,811, which is a continuation ofapplication Ser. No. 268,387, filed on May 29, 1981, now abandoned.

The invention relates to a process for restoring or maintaining theactivity of heterogeneous catalysts for reactions at normal and lowpressures, which can lose their activity as a result of the deposition,physisorption or chemisorption of organic or inorganic substances whichare carried over into the reaction system or are formed in a chemicalsynthesis as a consequence of side reactions.

Losses of activity in heterogeneously catalyzed syntheses are of verygreat practical importance. A loss of catalytic activity results indecreased production output, and a decrease also by virtue of thedeterioration in yield and selectivity. In addition to its specificcatalytic action, and to the high mechanical strength required of acatalyst in practical operation, an activity which remains constant foras long a period as possible has, above all, a decisive effect on thecost efficiency of a process. Generally the cause of a catalytic loss inactivity, is aside from electronics effects, considered to be the resultof a reduction in the number of active sites on the internal or externalsurface of the catalyst which is primarily due to one of the following.

1.Poisoning caused by specific catalyst poisons which have been carriedover into the reaction system and which are so strongly physisorbed orchemisorbed on the active centers that they can no longer be desorbedsufficiently fast under the synthesis conditions normally employed.

2. The deposition on the active centers of the external or internalsurface of the catalyst ("fouling") of sparingly volatile substanceswhich (a) are carried over into the reaction system or (b) are formed asthe result of undesired parallel reactions or secondary reactions.

3. Aging caused by structural changes on the catalytically activesurface (for example sintering processes, recrystallization processes,transport reactions and the like).

The art contains numerous processes, mostly very involved processes, forprolonging the life of heterogeneous catalysts. For example, additionalprocess stages are frequently used, such as a preliminary purificationof the starting materials or process gases or circulatory gas washing,in order to suppress poisoning or deposition caused by substancescarried over into the reaction system. In general, however, effects ofthis type cannot be excluded completely, above all because of the highcosts associated therewith. Similarly, deposits are formed because ofside reactions industrial syntheses in which reactions are generallycarried out in highly concentrated systems and under drastic operatingconditions and with optional conversions in order to achieve highspace-time yields.

In many catalytic syntheses in organic chemistry, particularly inpetrochemistry, high-molecular polymerization products which are rich incarbon are deposited on the catalysts. This is described quite generallyas coking, but the rate and the extent of the deposition of coke arevery different in the various processes. In catalytic cracking, forexample, it is necessary after as short a period as hours or fractionsof an hour to reactivate the catalyst, that is to say to restore itsactivity, so that the process is frequently carried out in severalreactors which are operated alternately. If the catalyst is in a fixedbed, the reactivation is in most cases carried out by slowly burning offthe coke, in which process it is necessary to ensure carefully that atemperature limit harmful to the catalyst is not exceeded and that agingas described above caused by heat does not take place. Moving bed andfluidized bed catalysts are frequently reactivated by withdrawing partof the catalyst from the reactor and calcining it in a regenerator, thesize of which frequently exceeds that of the reactor itself. In thisrespect, the desire to burn off coke as quickly as possible since rapidburning off can be carried out even in a small regenerator must bebalanced by the requirement that the coke must be burnt off gently.

Kinetic research described in the literature concerning the mechanism ofthe deactivation of heterogeneous contact catalysts by fouling resultingfrom side reactions is chiefly concerned with the formation of coke incracking reactions. It is known from these investigations that thecontent of aromatics and olefins has a great effect on the extent andrate of coking. Particularly in the case of reactions involving olefinson acid contact catalysts, polymerization reactions are suggested as theorigin of the deactivation process. Because of their high molecularweight and their fairly low volatility, the oligomers initially formedare no longer desorbed from the contact catalyst at the same rate atwhich they are formed and they cause deposition of high-molecularmaterial through further reactions.

It was indeed already known that increasing the reaction temperature canpromote the desorption and the detachment of sparingly volatilesubstances in some cases. In general, however, the advantages ofimproved desorption conditions at higher temperatures are eliminated bythe disadvantages of an increased reaction rate for side reactions. Inregard to the effect of pressure in deactivation processes, reference ismade in the literature to the fact that a higher pressure promotessorption processes and thus accelerates the loss of activity caused bythe deposition, physisorption or chemisorption of deactivatingsubstances.

It has been found that the removal of such substances from the catalystor prevention of initial deposition or formation can be improvedsignificantly in accordance with the present invention.

Substances which have already been deposited or absorbed are removedfrom the catalyst in a very gentle manner and are removed from thereaction system. If supercritical reaction conditions are selected fromthe outset, compounds which have been carried over into the reactionsystem or which are formed in the course of side reactions are preventedfrom becoming deposited or absorbed and are discharged continuously.

The process according to the invention is relevent, not only in the caseof high-molecular deposits which are rich in carbon, but also in thecase of catalyst poisons having a specific action which are physisorbedor chemisorbed on the reactive centers. The pressures to be used arefrequently already within the pressure range from approx. 50 to approx.150 bars which is of industrial interest. In cases where depositedproducts can react further (for example polymerize), it is possible forsuch a further reaction to be completely suppressed. The more rapidlythe reactivation process is to be effected and the more sparinglyvolatile the deactivating substances or the more strongly they arephysisorbed or chemisorbed, the higher is the pressure required. Thisprocedure also makes possible partial removal from the catalyst, wherebythe conditions can be adjusted so that, for example, catalyticallyactive substances (for example noble metals) which have been depositedon a support material, remain on the catalyst and only deactivatingsubstances are removed. The procedure can be carried out by choosing atemperature equal to or higher than the critical temperature (T≧T_(c))and varying the pressure (p>p.sub. c), or by selecting a pressuregreater than the critical pressure of the fluid phase (p>p_(c)) andvarying the temperature (T>T_(c)).

Compared with known methods for reactivating heterogeneous catalysts,the process according to the invention provides the followingsubstantial advantages:

(A) It is carried out under relatively mild conditions. Compared withthe method of reactivating catalysts for calcining, the risk ofdeactivating the catalyst through aging is avoided.

(B) The restoration or maintenance of the activity is effected throughthe fluid reaction phase itself. In this way no foreign substances arecarried over into the reaction system, as is the case, for example, withextractive purification.

(C) The process can be carried out "in situ" and does not cause anylengthy periods of down-time.

(D) The process according to the invention makes it possible to effect afractional removal of deactivating substances.

If a pressure which is at least equal to the critical pressure and atemperature which is at least equal to the critical temperature of thefluid reaction phase are applied throughout the duration of thereaction, this produces the following additional advantages:

(E) The heterogeneous catalyst employed in a chemical syntheses can beoperated for very long reaction times at a constant catalytic activity.

(F) If the catalytic activity remains constant, it is possible to employconsiderably more drastic conditions in the catalytic reaction and thusachieve higher space-time yields.

If the critical pressure or the critical temperature of the fluid phasehave particularly high values, it is frequently advisable to reducethese values by adding auxiliary substances. Examples of suitableauxiliary substances are hydrocarbons, such as methane, propane, butaneor pentane, or carbon dioxide or nitrogen or argon.

The process according to the invention is applicable, for example, tothe isomerization of 1-hexene on oxide contact catalysts such as η-Al₂O₃, the alkylation of benzene with olefins, such as ethylene, propene,butene, n-hexene or cyclohexene, on oxide contact catalysts such asη-Al₂ O₃ or zeolites, and to isomerization reactions of saturatedhydrocarbons and to cracking, dehydrogenation, halogenation andsulfonation reactions. Examples of other suitable fields of applicationare reactions in which addition or polymerization reactions result inundesirable higher-molecular compounds, such as oxyalkylation reactionsof alcohols, phenols or amines by means of ethylene oxide or propyleneoxide.

The following examples illustrate the invention:

EXAMPLE 1 (Catalyst deactivation caused by the deposition of sparinglyvolatile substances formed as the result of parallel or secondaryreactions; FIG. 1)

7.5 g of freshly prepared catalyst (η-Al₂ O₃ ; particle diameter:0.6-0.75 mm; catalytically active specific surface area: 4.95 m² /g) areintroduced, in an activated state, into a high-pressure differentialcirculating reactor having a fixed bed arrangement for the catalyst.1-Hexene containing 0.016 mole/l of 2-chlorohexane is fed continuouslyinto the reactor at a pressure of 15 bars and a temperature of 250° C.(average residence time: 1.08×10⁵ s.g/mole; circulation factor: approx.100).

As shown by curve 1 in FIG. 1, under these conditions, with a gaseousfluid phase, the 24% conversion of 1-hexene expected from theory (curve2) is not achieved after stable operating conditions have beenestablished in the circulating reactor (approx. 4 hours=four times theaverage residence time). The resulting course of the reaction is typicalof a reaction on which a deactivation process is superimposed. In thecolorless solution of product which emerges, only the isomeric hexenes(1-hexene, cis-2-hexene, trans-2-hexene and trans-3-hexene) can bedetected by gas chromatography (GC) and mass spectroscopy (MS) and byanalysis by capillary gas chromatography.

Establishing supercritical conditions by increasing the pressure to 500bars, after an operating time of about 14 hours, produces an immediatedark brown discoloration of the product solution, which graduallybecomes pale again. The conversion after this reactivation phasecorresponds to the figure to be expected on the basis of theory and, asillustrated in curve 3, no decrease in the catalytic activity isobserved under these conditions, even over extremely long operatingtimes, in spite of the considerably higher reaction rate which isproduced.

In the product solution with emerges with a dark brown discoloration, itis possible to detect by GC/MS methods the corresponding hexeneoligomers (C₁₂, C₁₈, C₂₄ . . . compounds) which, in the isomerizationreaction employing a gaseous reactant phase, remain on the catalyst byvirtue of their low volatility and cover the active centers. A constant,small quantity of hexene oligomers can be determined in the productsolution which emerges after the reactivation phase.

EXAMPLE 2 (Catalyst deactivation caused by a specific catalyst poison;FIG. 2)

A 6.4×10⁻² molar solution of pyridine in 1-hexene is fed continuouslyfor about 12 hours into a differential circulating reactor having afixed bed arrangement corresponding to Example 1, at a temperature of45° C. and a pressure of 50 bars. After the active acid centers of thecatalyst have been poisoned by pyridine, the conversion in theisomerization reaction rapidly falls nearly to zero, corresponding tothe deactivation illustrated in curve 1 of FIG. 2.

The catalyst can no longer be reactivated, even if very long operatingtimes are employed, by increasing the temperature to 220° C. and thepressure to 500 bars, that is to say at sub-critical conditions and aliquid fluid phase.

After increasing the temperature further to 250° C., with the pressureunchanged at 500 bars, that is to say with a supercritical fluid phase,when stable operating conditions have been established, the conversionachieved is initially only approx. 8% and this increases slowly withtime, in accordance with curve 2, until finally, after about 24 hours,the original figure of 40% conversion is re-established.

Pyridinium hydrochloride can be detected in the product solution whichemerges during the reactivation phase.

EXAMPLE 3 (Catalyst deactivation caused by sparingly volatile inorganicsubstances carried over into the reaction system; FIG. 3)

1-Hexene containing small quantities of a lubricant based on MoS₂ isintroduced into a differential circulating reactor having a fixed bedarrangement for the catalyst corresponding to Example 1. A loss inconversion corresponding to curve 1 in FIG. 3 is determined for thehexene isomerization reaction at a temperature of 220° C. and a pressureof 500 bars and a liquid fluid phase. Deactivation of the catalystthrough deposition of the lubricant is superimposed on the isomerizationreaction. Under these conditions, the catalyst can no longer bereactivated even after extremely long operating times. Increasing thetemperature to 240° C., with the pressure unchanged at 500 bars, that isto say with a supercritical fluid phase, produces the reactivation shownin FIG. 3. The catalyst recovers its full activity after about 75 hours.Curve 2 for the isomerization reaction in the absence of deactivation isobtained using the catalyst which has been reactivated in this way.

EXAMPLE 4 (Maintaining the activity of the catalyst in a synthesis asthe result of selecting supercritical reaction conditions; FIG. 4)

A freshly prepared catalyst is introduced, in an activated statecorresponding to Example 1, into a differential circulating reactorhaving a fixed bed arrangement for the catalyst. After stationaryconditions have been established at a temperature of 250° C. and apressure of 75 bars (curve 1), 250 bars (curve 2), 500 bars (curve 3) or850 (curve 4), a conversion which remains constant even over extremelylong experimental periods is obtained for the isomerization of 1-hexene.

Small quantities of hexene oligomers can be detected by GS/MS methods inthe product solutions which emerge.

EXAMPLE 5 (Catalyst deactivation caused by the deposition of sparinglyvolatile substances formed in parallel or secondary reactions)

5 g of freshly prepared catalyst, in an activated state corresponding toExample 1, are introduced into a differential circulating reactor havinga fixed bed arrangement for the catalyst. Benzene, cyclohexene and2-chlorohexane in a molar ratio of 600:200:1 are fed continuously intothe reactor at a pressure of 15 bars and a temperature of 305° C.

Under these conditions, in which the fluid phase is gaseous, theconversion efficiency, relative to the benzene employed, achieved afterstationary operating conditions have been established in the circulatingreactor (approx. 4 hours=four times the average residence time) is notconstant. Deactivation of the catalyst is superimposed on the alkylationreaction. After an operating time as short as 20 hours, the degree ofconversion falls by approx. 30% from the maximum figure obtainable.

Increasing the pressure to 150 bars to give a supercritical fluid phaseproduces an immediate dark brown discoloration of the product solution,which gradually becomes pale again. The conversion established after thereactivation phase at the higher pressure and with the operatingconditions otherwise identical, remains constant over long periods ofoperation; it is about 40% higher than the conversion beforereactivation.

We claim:
 1. A process for reactivating or maintaining the activity ofan oxide contact catalyst useful for catalytic alkylation of benzenewith an olefin at atmospheric or low pressure wherein the oxide contactcatalyst tend to decrease in activity by deposition, physisorption orchemisorption of organic or inorganic deactivating substances present atthe start of said catalytic alkylation or formed during said alkylation,which comprises conducting said catalytic alkylation at an operatingpressure above the critical pressure and a temperature of at least equalto the critical temperature of the fluid benzene and olefin reactantsfor a period of time at least sufficient to remove any deactivatingsubstances on said catalyst and to avoid formation, deposition,physisorption or chemisorption thereof on said catalyst.
 2. The processof claim 1 wherein said operating pressure and temperature are selectedto effectuate fractional removal of deactivating substances present onsaid catalyst.
 3. The process of claim 1 wherein the effective criticalpressure and temperature of said benzene and olefin reactants is loweredby addition of an effective amount of a compatible reaction auxiliaryinert to said catalytic alkylation.
 4. The process of claim 1 whereinsaid catalyst is η-Al₂ O₃.
 5. The process of claim 1 wherein saidcatalyst is a zeolite.
 6. A process for reactivating or maintaining theactivity of an oxide contact catalyst useful for catalytic alkylation ofbenzene with an olefin at atmospheric or low pressure wherein the oxidecontact catalyst tends to decrease in activity by deposition,physisorption or chemisorption of organic or inorganic substancespresent at the start of said catalytic alkylation or formed during saidalkylation, which comprises conducting said catalytic alkylation at anoperating pressure and temperature at least equal to the criticalpressure and temperature of the fluid benzene and olefin reactants for aperiod of time at least sufficient to remove any deactivating substanceson said catalyst and to avoid formation, deposition, physisorption orchemisorption thereof on said catalyst.
 7. The process of claim 6wherein the effective critical pressure and temperature of said benzeneand olefin reactants is lowered by addition of an effective amount of acompatible reaction auxiliary inert to said catalytic alkylation.
 8. Theprocess of claim 6 wherein said catalyst is η-Al₂ O₃.
 9. The process ofclaim 6 wherein said catalyst is a zeolite.